diff --git a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
index 2251d3ac63..4b4cb0a537 100644
--- a/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/mail/PhabricatorMetaMTAMail.php
@@ -88,11 +88,13 @@ class PhabricatorMetaMTAMail extends PhabricatorMetaMTADAO {
   }
 
   public function addTos(array $phids) {
+    $phids = array_unique($phids);
     $this->setParam('to', $phids);
     return $this;
   }
 
   public function addCCs(array $phids) {
+    $phids = array_unique($phids);
     $this->setParam('cc', $phids);
     return $this;
   }